The invention relates to a method for monitoring the length of constant-wire-length bonds employed on a circuit board, in particular, but not exclusively, circuit boards for microwave-circuit use.
It is often required in circuit-board construction to interconnect two components (e.g., two thin-film substrates, or a thin-film substrate and an active electronic component, e.g., a MMIC), disposed adjacent to each other on a common carrier. Where the interconnections are intended to carry RF (e.g., microwave) signals, it is important that the interconnections have predictable electrical characteristics (parasitic inductance and capacitance). Unfortunately, interconnection techniques commonly employed at sub-RF frequencies give rise to variable parasitic parameters due to the fact that certain tolerances are involved in the construction of the circuit-board. Thus, for example, the placing of the substrates (or substrate and active component) on the carrier will have an accuracy-tolerance, as will also the disposition of the interconnect bond-pads on the substrates relative to the edges of the respective substrates. Since pad-spacing will vary, the simpler bonding techniques will result in interconnects (bond-wires) of different length between the bond-pads of the two substrates (or of the substrate and active component) and these different lengths will have associated with them different inductances, which is undesirable.
To counteract this, it is known, especially in the microwave field, to employ a constant-wire-length (CWL) bonding system for the bonding together of the components on the carrier. To achieve this, the preferred bonding technique is normally the so-called xe2x80x9cball and wedgexe2x80x9d or, alternatively, the xe2x80x9cwedge and wedgexe2x80x9d bonding method.
Use of a CWL technique especially at microwave frequencies has the beneficial effect of ensuring that the electrical characteristics of all the RF-signal-carrying bonds on the circuit board are substantially the same. A representative CWL bond is shown in FIG. 1. In FIG. 1 a circuit-board arrangement 10 comprises a couple of substrates 11, 12 secured to a housing or carrier 13 by means of an adhesive 14, and a 50-ohm line 15, 16 on each of the substrates. Connecting the two lines are wire-bonds 17 formed by the ball-and-wedge method. Under this method a continuous length of wire is attached at its lower end to a first one of the sites 18 by a gold ball under the application of ultrasonic energy, is looped over to the corresponding one of the sites 19 and is attached, again via ultrasonics, to this site and thereby also to the line 16. The second attachment of this pair of attachments is formed as a wedge shape, hence the name of the process: xe2x80x9cball-and-wedge bondingxe2x80x9d. The wire loop is then detached from the second bond and the new wire-end used to start the next bonding operation.
The loops 20 are of constant length, not only for the illustrated bonds, but also for every other RF bond on the circuit board. This allows all the RF bonds to have a more or less constant electrical characteristic by virtue of the fact that the constant-length wires are of constant inductance. This inductance in conjunction with the fixed parasitic capacitance associated with the various line pads (the pads are designed to have a substantially equal capacitance characteristic) forms a low-pass L-C filter 22 which affects the signals carried by the lines in a similar and predictable way. The length of the bonds is chosen to accommodate the maximum anticipated inter-pad spacing (taking into account tolerances), which means that those bonds which are associated with less-than-maximum pad spacings will have a marked curve as shown in FIG. 1.
Unfortunately, it has been found difficult in practice to predict the exact length that the various bond-wires will possess during a manufacturing run. While the wires will normally all be of substantially constant length, the exact value of that length will possess a degree of uncertainty due to unavoidable tolerances in the bond-making process. (Such tolerances include those associated with capillary movement and wire-clamping and those associated with that part of the bond-making system which recognizes the bond-pad pattern on the substrates, etc). Consequently, there is a need for a method of monitoring and controlling the constant wire-length such that that length approaches a desired value. Preferably, such a method would achieve this requirement in a non-invasive way as far as the circuit-related bonds, i.e., those bonds which play a part in the relevant electronic circuitry, are concerned.
According to a first aspect of the invention, there is provided a method for monitoring the length of constant-wire-length (CWL) bonds employed on a circuit-board.
According to a second aspect of the invention, the invention provides a method for controlling the length of constant-wire-length (CWL) bonds employed on a circuit-board.
In a third aspect, the invention provides a method for monitoring the length of a wire-bond employed on a circuit-board.
In a fourth aspect, the invention provides for an arrangement for monitoring the length of one or more CWL wire-bonds employed on a circuit-board.